Photocurable resins, which cure through an ultraviolet ray or an electron beam, have short curing time and contain no solvents, and thus have advantages, such as resource saving performance, energy saving performance, and high productivity, over thermosetting resins. For this reason, photocurable resins are used for a variety of applications such as coating materials, coating agents, hard coating agents, inks, adhesives, pressure-sensitive adhesives, resists, molding materials, surface treatment agents, and surface modifiers. Acrylates and methacrylates are widely used for resin compositions containing such a photocurable resin because of the satisfactory curability (NPL 1).
In the resin composition, a diacrylate monomer or a dimethacrylate monomer is blended as a reactive diluent or a viscosity modifier. Examples of the diacrylate include 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,9-nonanediol diacrylate, neopentyl glycol diacrylate, ethylene glycol diacrylate, polyethylene glycol diacrylate, dipropylene glycol diacrylate, and tripropylene glycol diacrylate. Examples of the dimethacrylate, in which the acrylate moiety of the diacrylate is replaced with methacrylate, include 1,4-butanediol dimethacrylate, 1,6-hexanediol dimethacrylate, 1,9-nonanediol dimethacrylate, neopentyl glycol dimethacrylate, ethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, dipropylene glycol dimethacrylate, and tripropylene glycol dimethacrylate.
PTL 1 discloses that a coating agent the composition of which contains dineopentyl glycol diacrylate as a diluent at a content of 46% by mass based on a polyester acrylate consisting of adipic acid and phthalic acid, ethylene glycol and trimethylolpropane, and acrylic acid exhibits satisfactory adhesion and hardness.
However, neopentyl glycol diacrylate, 1,6-hexanediol diacrylate, neopentyl glycol dimethacrylate, 1,6-hexanediol dimethacrylate, and the like, which are commonly used, have high curing shrinkage, and thus have problems of lowering of flexibility and curling in curing. In addition, these diacrylates generally have a problem of high skin irritancy (NPL 1, NPL 2).
To reduce curing shrinkage and skin irritation, in many cases a diol as a raw material is modified into an alkylene oxide, and the modified product is converted into an acrylate for use. However, this method has a problem of lowering of curability concomitant with increase of viscosity by chain elongation (PTL 2). Moreover, use of such a modified product may lower the adhesiveness or the weatherability (NPL 1).